[time-nuts] 60Hz mains clocking in computers

[Joe Gwinn]

At 1:44 AM +0000 12/13/09, time-nuts-request at febo.com wrote:
>Date: Sat, 12 Dec 2009 17:29:17 -0800
>From: Colby Gutierrez-Kraybill <colby at astro.berkeley.edu>
>Subject: [time-nuts] 60Hz mains clocking in computers
>To: time-nuts at febo.com
>Message-ID: <3058527A-CC99-4174-BE75-21DD92334155 at astro.berkeley.edu>
>Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
>
>
>I'm trying to get to the bottom of whether or not any computing 
>equipment made around the advent of UNIX systems (or any time-slicing 
>system) used the mains cycles of 60Hz as phase lock for the internal 
>system clock.  My guess is that perhaps they did not as the computing 
>logic is DC based, but, I have memories of using an 68000 based UNIX 
>system that I thought had its internal clock based off of the 60Hz
>mains...  Not sure the vendor anymore.

In the 1980s and 1990s, before networks capable of carrying NTP time 
to the millions became common, the computer local clock was very 
often derived from the local AC power mains, and the frequency was 
steered to match atomic time once per day.  The POSIX standards 
reflect this common approach by the tolerance on CLOCK_REALTIME, 20 
milliseconds, this being one cycle of 50 Hz power.

The CPU logic clock was not generally phase-locked to the AC power 
lines, instead being generated by a cheap crystal having a very large 
tempco.  The exception to this was that video generators were (and 
still are) often locked to the AC line so that hum bars would not 
drift across the screen.

Joe Gwinn